1. Field of the Invention
The present invention relates to a field/frame conversion method in magnetic picture recording and, more particularly, to an improved field/frame coversion method which is capable of preventing the reduction of an SN ratio and the occurrence of flicker resulting from the delayed operations of field signals.
2. Description of the Prior Art
In scanning a television screen, a system of so called interlaced scanning is used in which horizontal scanning lines are interlaced on a given number of lines, in order to be able to reduce flicker to the eyes. Generally, a specific system of interlaced scanning, in which the successive scanning lines are interlaced on every other line (2:1), is employed. In the (2:1) interlaced scanning system, a rough picture (i.e., a field) is produced in each vertical scanning and two of such rough pictures are combined to form a complete picture (i.e., a frame). A field frequency is, in the NTSC system, for example, 60 Hz, while a frame frequency is 30 Hz, 1 frame is generally expressed by 525 horizontal scanning lines. Also, odd fields and even fields are shifted in the points of initiation of horizontal scanning thereof by half the horizontal scanning period (H) or by 0.5H in relation to one another.
When video signals are to be recorded on a magnetic tape, a magnetic disc, or other various recording mediums, in general, a 1-field signal or a 1-frame signal is allocated per track. Also, in the 1-field/1-track recording, there are available two kinds of recording systems: that is, one is a so-called 1-frame/2-track recording system in which the odd and even fields are recorded successively; and, the other is a field recording system in which only either of the odd fields or the even fields are recorded. In recording, generally, at least a brightness signal Y of the video signals is pre-emphasis processed or pre-emphasized before it is frequency modulated. Now, FIG. 3 illustrates a schematic circuit diagram of a recording system in an electronic still camera employing a magnetic disc. Specifically, in FIG. 3, a brightness signal Y in a base band is passed through a pre-emphasis circuit 1 and is frequency modulated on the high frequency side thereof by a frequency modulator 2. On the other hand, two color difference signals R-Y, B-Y in the base band are selected alternately at each horizontal scanning period (1H) and line-sequentialized by a switch 4, are passed through a pre-emphasis circuit 6, and are then frequency modulated on the low frequency sides thereof by a frequency modulator 7. The resultant FM brightness signal Y.sub.FM and FM line sequential color difference signal C.sub.FM are respectively passed through amplifiers 3, 8, mixed together, and then supplied to a magnetic head 9. In FIG. 3, the numeral 5 designates a control signal for the switch 4. Also, the frequency modulation of the color difference signals R-Y, B-Y is performed using different center frequencies so as to be able to distinguish them from each other.
For reproduction in the field recording system, the strong vertical correlation of the video signals is utilized and a so called field/frame conversion system is often used in which a frame signal is created from one kind of field signals by scanning the same track twice. This field/frame conversion system aims mainly at enhancing a recording density and thus, in this system, long time recordings are possible for moving pictures while the number of frames can be increased for still pictures. However, when converting the field signal into the frame signal, the interlaced scanning cannot be realized by simply repeating one and the same field signal twice, because such simple repetition of the same field signal is not able to produce the shift of 0.5H while the interlaced scanning requires a given time relationship between the vertical synchronizing signals and the horizontal synchronizing signals and video signals in the respective lines, that is, the odd and even fields must be shifted 0.5H from each other.
Therefore, as shown in FIG. 4, the same field signal 10 reproduced by repetition is passed through a 0.5H delay line 16 after it is demodulated and de-emphasized. In other words, the field signal 10 that is fed direct without passing through the 0.5H delay line and a field signal 17 that is passed through the 0.5H delay line and thus delayed by 0.5H are selected alternately at each vertical scanning period (1 V) by an analog switch 18, so that the field signal 10 is converted into a frame signal 19. If this operation is kept on like that, then intervals between the vertical synchronizing signals will be shifted 0.5H from 1 V. To compensate this, for example, selection of contacts a, b of the analog switch 18 may be performed in a manner shown in FIG. 5. That is, only within a region 21 ranging from a front equalization section to a back equalization section out of a period to select the non-delayed field signal 10, the 0.5H delayed field signal can be selected. At any rate, in order to convert the field signal into the frame signal, as shown in FIG. 4, a circuit is used which is capable of selecting the non-delayed signal and the 0.5H delayed signal.
In FIG. 4, 11 designates a magnetic head, 12 a playback amplifier, 13 a limiter, 14 an FM demodulator, 15 a de-emphasis circuit, and 20 a control signal for the switch 18.
When the field signal in the base band is delayed in this manner, the field signal passing through a delay circuit always receives noise from the delay circuit, with the result that the SN ratio thereof is decreased 1-2 dB than that of the non-delayed field signal and the delayed signal may be attenuated by the delay circuit.
In other words, there is produced a difference between the levels of the field signal taken directly from the de-emphasis circuit 15 and the the field signal delayed through the 0.5H delay line 16, which difference gives rise to flicker on a display screen. In order to prevent the occurrences of the flicker, an amplifier may be provided after the delay line 16 to adjust the different levels. However, in this case, the level adjustment is affected after all by the temperature characteristics and secular changes of the amplifier and delay line, so that the flicker is unavoidable. Especially when the delay circuit employs a charge coupled device (CCD) delay element, due to the structure thereof, there is produced noise such as a clock pulse or the like which comes in from the outside of the circuit, and at the same time there are produced various kinds of noise due to thermal noise occurring within the charge coupled device, trap noise, or other causes, thereby reducing the SN ratio of the delayed field signal to a great extent.
Therefore, in order to prevent the occurrence of flicker caused by the reduction of the SN ratio of the field signal, it is necessary to provide an automatic gain control (AGC) circuit which permits a high accuracy of gain control to adjust the level of the field signal that has passed through the delay circuit, which results in a complicated circuit configuration in the reproduction system.
Also, when the field/frame conversion is carried out in the above-mentioned manner, a V jitter is generated in a picture reproduced on the television screen.
This V jitter means a displacement of the picture on the TV screen in the vertical direction (V direction) thereof, the displacement having a width corresponding to one horizontal scanning line (1H) to be repeated at a field period on the television picture. That is, in this case, the picture on the television screen is caused to displace in the vertical direction at each field period (1/60 sec.) as wide as 1H.
As effective means to prevent such V jitter, there is conventionally employed an operation to find the arithmetic mean of the brightness signals of the odd fields. In other words, the brightness signal delayed by 1 horizontal scan period and the undelayed or non-delayed brightness signal are added, the sum is divided by 2, and the resultant value is used as a brightness signal in the odd fields.
In this case as well, however, the SN ratio of the brightness signal is similarly lowered through the 1 horizontal scan period delay operation.